for a d300 which is cropped, vignetting won't be a problem for either at 24mm.

it them comes down do to do you want to be able to use a lense cap? You can't with the thin one.

the thicker one is also coated, which may or not be important to you. I've not found that it really doens't make too much of a deal with a polarizer, as long as it's a good one to start with, and the B&W ones are superb.

Joe,
The ordinary one won't vignette on 24-70 on FX and of course not on DX. However, if you want to get an UWA later, then perhaps get the slim one as you can use it on the UWA, if and when you get it.